bioenergetics (1)

8/8/2019 BIOENERGETICS (1)

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BIOCHEMISTRY

JULIUS P. MARIO, RMT, MSci


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ENERGY

Capacity to do work

unlike matter, it is known and recognized byits effects

cannot be seen, touched, smelled or weighed

constant in the universe


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TYPES OF ENERGY

Radiant = solar energy

Thermal = associated with random motion of

atoms and moleculesChemical = stored within the structural units of

chemical substances

= can be released, stored or convertedto other energy forms

Potential = available by virtue of an objects

position


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ENERGY CHANGES

chemical reactions absorb or release energy,

generally in the form of heat

HEAT

transfer of thermal energy between two bodies

that are at different temperatures


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THERMOCHEMIS

TRY

Study of heat change in chemical reactions

SYSTEM AND SURROUNDING

System = specific part of the universe that is of

interest to us

Surrounding = the rest of the universe outside the

system


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TYPES OF SYS

TEM

OPEN = can exchange mass and energy with its

surroundings

CLOSED = allows the transfer of energy but not

mass

ISOLATED = does not allow the transfer of

either mass or energy


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EXOTHERMIC PROCESS

= any process that gives off heat

= transfers thermal energy to the surroundings= total energy of products is less than the total

energy of the reactants

2H2 (g) + O2 (g) 2H2O(M) + energy


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ENDOTHERMIC PROCESS

= any process that requires heat to be applied to

the system by the surroundings

= total energy of reactants is less than the total

energy of products

energy + 2HgO(s) 2Hg(M) + O2 (g)


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THERMODYNAMICS

scientific study of the interconversion of heat

and other kinds of energy

study of the changes in the state of a system

provide useful guidelines for understanding the

energetics and directions of processes


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STA

TE OF

THE SYS

TEM

the values of all relevant macroscopic

properties, for examples, composition, energy,

temperature, pressure and volume


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SIGN CONVENTIONS FOR WORK & HEA

T

PROCESS SIGN

work done by the system on the surroundings -

work done on the system by the surroundings +

heat absorbed by the system from the

surroundings (endothermic) +

heat absorbed by the surroundings from

the system (exothermic) -


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LAWS OFTHERMODYNAMICS1ST LAW

= energy cannot be created nor destroyed but can be

converted from one form to another

2ND LAW

= the entropy of the universe increases in a spontaneous

process and remains unchanged in an equilibrium

process

3RD LAW

= entropy of a perfect crystalline substance is zero at the

absolute zero temperature


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Reaction Rates and Chemical Equilibrium

Chemical kinetics = study of reaction rates

Reaction rate = change in concentration of a

reactant or product per unit time

molecular collisions - effective collisions

activation energy - minimum energy necessary

for the reaction to happen


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Depends on

1. Speed of colliding objects

2. Angle of approach = head-on is best3. Must have proper orientation

energy collision of activation energy - no

reactionH2O(M) + HCl(g) H3O

+(aq) + Cl-(aq)


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Activation energy & Energy Diagrams

Activation E

E of productsE of reactants

E or rxn

Progress of reaction

energy

Transition state

Downhill reactions - exothermic


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Activation energy & Energy Diagrams

Activation E E of products

E of reactants

E or rxn

Progress of reaction

energy

Transisiton state

Uphill reactions - endothermic


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Transition state - top of the energy hill

- one or more original bonds are

partially broken or

- one or more bonds (new) may

be in the process of formation


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Activation energy - energy hill

- if low, faster reaction

- if high, slower reaction


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Factors Affecting Rates of Reaction

A. Nature of reactants for solid = surface area

affects for gases, pressure- rate ions in

aqueous solution - instantaneous

B. Concentration

direct relationship between rate and

concentration


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C. Temperature

= a 10oC rise in temp, reaction rate doubles

= has two reasons:

1.temp - more rapid movement of

molecules

- more probability of collision

2. Different distribution of speeds - more

effective collisions than total number of

collisions


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D. presence of catalyst - alternative pathway by

providing surface for molecules to meet

heterogeneous - separate phase from reactant

homogeneous - same phase as the reactant


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Once equilibrium is reached, the following equation is

valid

K = [C]c[D]d the equilibrium expression

[A]a[B]b

CO(g) + H2O(g) CO2(g) + H2(g)

K = [CO

2][H2] [x] = x in mol/L[CO][H2O] x = liquid or gaseous

Ab equilibrium, the concentration of CO2 x concentration

of H2 & divided by the concentrations of H2O & CO isa constant, K.

Universal custom is to write them with Products on top and

Reactants below.


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Write the equilibrium expression for the reaction

SO3(g) + H2O(M) H2SO4(g)

O2(g) + 4Cl2(g) 2Cl2O5(g)

2NH3(g) N2(g) + 3H2(g)

I2(g) + H2(g) 2HI(g)

PCl3 + Cl2 PCl5


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Le Chateliers Principle

When a reaction equilibrium, the forward and

reverse reactions take place at the same rate,

and the concentrations of all components do

not change as long as we dont do anything to

the system.


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If an external stress is applied to a system in

equilibrium, the system reacts in such a way as to

partially relieve that stress.

A. Addition of a Reaction Component

HCl - catalyst-doesnt affect equi

CH3COOH + C

2H

5OH CH

3COOC

2H

5+ H

2O

acetic acid ethyl acetate

= results to equilibrium shift


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B.Removal of a Reaction Component

reaction rate but not easy to do

NO MATTER WHAT HAPPENS TO THE

INDIVIDUAL CONCS, THE VALUES OF

THE EQUILIBRIUM CONSTANT (K)REMAINS UNCHANGED.


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C. Change in Temperature

The effect of a ( T on a reaction that has

reached equilibrium depends on whether the

rxn is exothermic (gives off heat) or

endothermic (required heat).2H2(g) + O2(g) 2H2O(M) + 137,000 cal

temp + heat

heat as product, its addition pushesthe equilibrium to the opposite

siteto the left

H2 & O2; & H2O


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in T drives an exo rxn towards reactants (left)

in T drives an exo rxn towards product (right

for endo, opposite is true)

= changes both the position of equilibrium but also

the value of K, (equi contant)


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D.Change in pressure= influences the equilibrium only if one or more of

the components of the reaction is a gas

N2O4(g) 2NO2(g)

one mole of gas 2 moles of gas(reactant) (product)

= in P shifts the equi in the direction that will the

moles in the gas phase and hence the pressure.

With the above, the shift is to the left.

An increase in pressure shifts the reaction toward

the side with fewer moles of gas (otherwise if

theres in P).


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E. Effect of catalyst

= the rates of both the forward and reverse

reactions to the same extent.

: addition of catalyst has no effect on the positionof equilibrium.

However, adding catalyst to a system not yet at equi

caused it to reach equi faster than it would without

the catalyst.


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G and Go

G = Gibbs free energy

= energy available to do work

G = change in free energy

G < 0 spontaneous in the forward reaction

G > 0 nonspontaneous; spontaneous in theopposite direction

G = 0 at equilibrium; no net change


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Go = standard free-energy of reaction

= free-energy change for a reaction when

it occurs under standard-state conditions,

when reactants in their standard states are

are converted to products in their standardstates.

GAS = 1 atm pressure

LIQUID = pure liquid

SOLID = pure solid

ELEMENTS => Go f= 0

SOLUTION = 1 molar concentration


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Free energy is a useful thermodynamic function

for understanding enzymes

Some of the principles of thermodynamics:

To fully understand how enzymes operate, we need to

consider two thermodynamic properties of the reaction:

(1) the free-energy difference ((G) between theproducts and reactants and

(2) the energy required to initiate the conversion of

reactants to products.

The former determines whether the reaction will be

spontaneous, whereas the latter determines the rate of

the reaction. Enzymes affect only the latter.


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The Free-Energy Change Provides Information

About the Spontaneity but Not the Rate of a

Reaction

The free-energy change of a reaction ((G) tells us if the

reaction can occur spontaneously:

1. A reaction can occur spontaneously only if(G is

negative. Such reactions are said to be exergonic.2. A system is at equilibrium and no net change can take

place if(G is zero.

3. A reaction cannot occur spontaneously if(G is positive.

An input of free energy is required to drive such areaction. These reactions are termed endergonic.


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Two additional points need to be emphasized.

=the (G of a reaction depends only on the freeenergy of the products (the final state) minus thefree-energy of the reactants (the initial state)

= the (G of a reaction is independent of the path(or molecular mechanism) of the transformation.The mechanism of a reaction has no effect on(G.

For example, the (G for the oxidation of glucoseto CO2 and H2O is the same whether it occurs bycombustion in vitro or by a series of enzyme-catalyzed steps in a cell.


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=the (G provides no information about the rate of a

reaction

A negative (G indicates that a reaction can occur

spontaneously, but it does not signify whether it will

proceed at a perceptible rate.

The rate of a reaction depends on the free energy of

activation ((G), which is largely unrelated to the (G of

the reaction.


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The Standard Free-Energy Change of a Reaction

Is Related to the Equilibrium Constant

As for any reaction, we need to be able todetermine (G for an enzyme-catalyzed reactionin order to know whether the reaction isspontaneous or an input of energy is required.

To determine this important thermodynamicparameter, we need to take into account thenature of both the reactants and the products aswell as their concentrations.

Consider the reaction

A + B C + D


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The (G of this reaction is given by

[C][D]

(G = (Go + RT ln [A][B]

where:

(Go is the standard free-energy change

R is the gas constant

T is the absolute temperature,

and [A], [B], [C] and [D] are the molar concentrations (more precisely, the

activities) of the reactants

(Go is the free-energy change for this reaction under

standard conditions, that is, when each of the reactants

A, B, C and D is present at a concentration of1.0 M (for

a gas, the standard state is usually chosen to be 1

atmosphere).


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Relation between (Go and Keqat 25oC

(Go

eq kcal mol-1

kJ/mol-1

10-5

6.82 28.53

10-4

5.46 22.84

10-3

4.09 17.11

10-2 2.73 11.4210

-11.36 5.69

1 0 0

10 -1.36 -5.69

102

-2.73 -11.42

10

3

-4.09 -17.1110

4-5.46 -22.84

105

-6.82 -28.53


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The overall free-energy change for a chemically

coupled series of reactions is equal to the sum of thefree-energy changes of the individual steps.

A - B + C Go = +5 kcal/mol

B - D Go = -8 kcal/mol

______________________________________

A - C + D Go = -3 kcal/mol

A thermodynamically unfavorable reaction can be

driven by a thermodynamically favorable reaction to

which it is coupled.


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Standardfree energies ofhydrolysis of

some phosphorylated compounds

Phosphoenolpyruvate -14.8 kcal/mol

1,3-bisphosphoglycerate -11.8 kcal/mol

Creatine phosphate -10.3 kcal/mol

ATP to ADP -7.3 kcal/mol

Glucose-1-phosphate -5.0 kcal/mol

Pyrophosphate (PPi) -4.6 kcal/mol

Glucose-6-phosphate -3.3 kcal/mol Glycerol-3-phosphate -2.2 kcal/mol

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